Frank McNab (or MacNab) was second in command of The Regulators, the Lincoln County (NM) outfit that tried to settle the score for the murder of cattleman/entrepreneur John Tunstall. Billy the Kid was one of his cohorts.
McNab was involved in the March 1878 killings of Frank Baker and Billy Morton, who allegedly participated in the Tunstall shooting. When Regulator William McCloskey objected, McNab shot him dead. He was later one of the assassins of Sheriff William Brady and Deputy George Hindman. But McNab got his soon after; he was killed by opponents on April 29, 1878.